This story appears in the Fall 2020 Letters & Science magazine.
Asked to imagine a world without plankton, Carol Lee shudders.
“Oh my God,” says the professor of integrative biology, who studies the ability of plankton to evolve in the face of rapid human-induced environmental change. “There would be algae, but no fish. It would be like getting rid of insects on land. It would be really bad.”
Plankton, which include the copepods and Daphnia that help keep toxic algae blooms at bay in Wisconsin lakes, are a diverse group of animals found in water bodies around the world. Their numbers are declining worldwide, with potentially devastating consequences for foodwebs and fisheries.
Last March, Lee was awarded a grant from the French government to study the capacity of plankton in the Baltic Sea (which is becoming warmer and fresher due to climate change) to evolve, in terms of the speed and extent of evolution. Hers was one of 12 projects worldwide to receive funding in a third phase of President Emmanuel Macron’s “Make Our Planet Great Again” initiative.
The project’s urgency is tied to the important role plankton play in food webs. Copepods are especially important because they form the largest biomass of all animals in the world’s oceans.
“They are like the cows of the sea,” says Lee. “They graze on algae, and they are a major food source for fish and whales.”
Over the past year, Lee has established a collaborative project at France’s Montpelier University and hired a graduate student, Theresa Popp, and a technician, Benjamin Kleinerman, to study the rapid evolutionary responses of plankton to salinity and temperature in the Baltic Sea.
“The Baltic Sea is a really important habitat for cod and herring,” says Lee. “But it’s like a semi-enclosed basin. It will become warmer and more like a freshwater lake. That will have devastating impacts on aquatic life.”
Lee wanted to know whether the plankton, at least, could adapt to changing conditions. If the basic food source remains, then there’s hope for sustaining fish populations—even if they differ from the ones that live in the Baltic Sea now.
Taking samples from the wild population, Lee’s team launched laboratory selection experiments to determine the extent to which this copepod could evolve to low salinity. What they found was that as salinity in the laboratory lines went down over a few generations, the selection process kicked in, targeting many of the same genes across replicate lines. The variants of these genes (alleles) that allowed for freshwater survival became more common (i.e., natural selection). This result indicated that evolution could operate in a predictable manner in response to rapid environmental change. Notably, these same alleles are present in the wild populations across the Baltic Sea, indicating that the alleles could be favored by selection in the wild as conditions get more fresh, and that the population has the capacity to evolve.
This is good news, says Lee, as many high-latitude water bodies are becoming more fresh, with ice melt and changes in precipitation. Other, larger animals with longer life cycles may not have time to adapt. But these copepods, with their 20-day generational cycle, have the evolutionary advantage over fish.
“They can get a lot of evolution done in one year,” says Lee, whose initial study from this project was published in the journal Nature: Ecology & Evolution this summer.